The present invention relates to exhaust treatment devices and more particularly relates to parallel plate non-thermal plasma reactors.
Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that they must be controlled in order to meet government emissions regulations. Among the regulated compounds are hydrocarbons, soot particulates, and nitrogen oxide compounds (NOx). There are a wide variety of combustion processes producing these emissions, for instance, coal- or oil-fired furnaces, reciprocating internal combustion engines (including gasoline spark ignition and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of these emissions are needed.
An alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust or effluent stream would be to destroy such emissions using a non-thermal plasma. Plasma is regarded as the fourth state of matter (ionized state of matter). Unlike thermal plasmas, non-thermal plasmas (NTPs) are in gaseous media at near-ambient temperature and pressure but have electron mean energies considerably higher than other gaseous species in the ambient environment. NTP species include electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These NTP species are highly reactive and can convert hazardous gases to non-hazardous or less hazardous and easily managed compounds through various chemical reaction mechanisms. In contrast to thermal processes (such as thermal plasma), an NTP process directs electrical energy to induce favorable gas chemical reactions, rather than using the energy to heat the gas. Therefore, NTP is much more energy-efficient than thermal plasma.
NTPs can be generated by electric discharge in the gas or injection of electrons into the gas by an electron beam. Electron beams must be accelerated under a high vacuum and then transferred through special windows to the reaction site. The reaction site must be sized with respect to the penetration depth of the electrons. It is much more difficult to scale-up the size of an electron beam reactor than an electric discharge reactor. Therefore, electron beam reactors are less favored than electric discharge reactors.
Among the various types of electric discharge reactors, pulse corona and dielectric barrier (silent) discharge reactors are very popular for their effectiveness and efficiency. However, pulse corona reactors have the major disadvantage of requiring special pulsed power supplies to initiate and terminate the pulsed corona. Consequently, dielectric barrier discharge has become a fast growing technology for pollution control.
Cylindrical and planar reactors are two common configurations for dielectric barrier discharge reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space. Other dielectric barrier discharge reactors include packed-bed discharge reactors, glow discharge reactors, and surface discharge reactors.
A variety of known dielectric barrier discharge NTP reactor designs are based upon the use of one or more structural dielectric ceramic pieces coated with a conductive material arranged to form the dielectric barrier-conductor-dielectric barrier configurations (xe2x80x9cparallel platexe2x80x9d reactors). Preparation of parallel plate NTP reactors typically involves pre-assembling the reactor stack by stacking alternating positive and negative plates, using ceramic spacers to separate and retain the plates. The ceramic spacers further serve to determine exhaust passage dimensions. A glass retaining material is typically applied at the reactor edges to produce a rigid one-piece assembly. The reactor assembly is in turn wrapped with a ceramic retention material, such as an intumescent mat, and stuffed into a cylindrical housing.
Problematically, the ceramic spacers must be individually placed and retained to separate the stacked plates. The lack of an insulating feature in the plate/spacer design requires separating the electrode from the edge of the dielectric by a wide margin, such as about 19 mm, to prevent surface conduction of electric energy through the gap of the plates thereby reducing overall reactor performance. Further, the ceramic spacers contribute to the overall height variation of the reactor assembly due to large thickness variation of the spacers, making it difficult to fit the assembly into the housing. The gap variations from cell to cell further reduce reactor performance by reducing or prohibiting plasma production resulting in so-called xe2x80x9cinter and or intra-dark cellsxe2x80x9d.
What is needed in the art is an improved non-thermal plasma reactor and method of producing same. What is further needed in the art is a NTP reactor that can be produced at reduced cost while providing improved reactor performance.
A non-thermal plasma reactor having individually retained plates and a method for preparing same is provided. One embodiment of the method comprises stacking an alternating sequence of positive and negative pairs of reactor plates to form a reactor stack having exhaust gas passages defined between opposing pairs of plates, using temporary spacers between opposing polarity pairs of said positive and negative reactor plates to support the stacked plates.
In a first embodiment, an insulating layer comprising, for example, a ceramic insulating fiber, is disposed on either side of the stack, and the stack is compressed. During compressing, the insulating layers fold a distance into the exhaust passage, providing insulating and support function. In this embodiment, the temporary spacers define the height of the exhaust gas passage. The reactor stack is wrapped with an insulating retention material, suitable electrical connections are provided to the plates, the temporary spacers are removed, and suitable inlet and outlet connections are provided to the reactor housing. The reactor plates in this embodiment are secured by the insulating retention material, enabling the plates to expand or contract independently of one another during operation. During operation, the reactor is powered with high voltage alternating current forming a non-thermal plasma in the exhaust passages for treating constituents present in an exhaust stream passing through the exhaust passages.
In a second, preferred embodiment, a permanent pleated insulating separator is disposed on each side of the stack, such that each pleat separates a pair of positive plates from a pair of negative plates. The temporary spacers have a thickness selected to effect a temporary spacing between the plates providing easy insertion of the permanent pleated insulating separators disposed on each side of the spaced reactor plates. In this preferred embodiment, the permanent pleated separators provide excellent durability by maintaining the spacing between the reactor plates in combination with a retention mat that is wrapped around the stack.
The temporary spacers are removed, and the stack is compressed to secure and compact the pleats of the permanent insulating separators. The permanent pleated insulating separators extend a distance into the exhaust passage, providing insulating and support function, and defining the exhaust gas passage height.
Suitable electrical connections may be made to the plates and the reactor stack is wrapped using a resilient retention material. The reactor assembly further comprises inserting the wrapped stack into a reactor housing by a stuffing operation, adding the required electrical connections and inlet-outlet connections to the reactor housing to prepare the present non-thermal plasma reactor. The reactor plates in this preferred embodiment are secured by the insulating retention material, and additional spacing and support function is provided by the permanent pleated insulating separators, enabling the plates to expand or contract independently of one another during operation.
In a third embodiment, an alternating sequence of positive and negative pairs of reactor plates are stacked to form a reactor stack having exhaust gas passages defined between opposing pairs of plates, using temporary spacers between opposing polarity pairs of said positive and negative reactor plates to support the stacked plates during preparation. A retention material, such as a ceramic fiber retention mat, is disposed about the stack. In this embodiment, permanent support for the stacked plates is supplied from the retention mat, which, upon compressing, extends slightly into the exhaust gas passages at each side of the stack.
Non-thermal plasma reactors prepared in accordance with the present method allow ease of manufacture at reduced cost due to the elimination of costly permanent ceramic spacers and edge assembly retention with glass cement. The reactor plates are individually retained, having the ability to expand and contract independently. Therefore, stack breakage due to cracking of the glass cement during assembly and plate breakage due to thermal stress caused by one plate expanding or contracting more than the other is greatly reduced or eliminated. Improved reactor performance is provided through elimination of the occurrence of xe2x80x9cdarkxe2x80x9d cells resulting from permanent ceramic spacers as well as through improved dimensional control. The present non-thermal plasma reactor and method provides improved dimensional control over stack height by using permanent pleated separators or insulating layers comprising an insulating material having very little thickness variation along the length of each piece.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.